This invention relates to interferometry, and more particularly to a speckle interferometer apparatus and method that utilizes a scattering reference plate that can incorporate phase shifting.
Optical interferometers are known which make use of the interference phenomena known as the "speckle effect," the speckled pattern seen when laser light is used to illuminate a rough surface. This invention utilizes the speckle effect, but it offers significant cost and performance improvements over conventional apparatus and methods.
U.S. Pat. No. 4,850,693 teaches a compact and portable moire interferometer for determining surface deformations of an object; and U.S. Pat. No. 4,794,550 teaches a method of extending the measurement range of the moire reference beam techniques by constraining the reconstruction of a surface contour based on a prior knowledge about the surface. These moire methods require that some form of a grating be created or projected onto the surface of the specimen, perhaps by the use of coherent laser light.
The physics of this invention are distinctly different from moire techniques. This invention measures deformations, displacements, and strains of an object, but it does not employ the "moire effect," in that no grating is created on the specimen or in the optical system. Only the "speckle effect" is used.
Speckle interferometry is known for use in measuring strain in structural members and mechanical components. U.S. Pat. No. 4,591,996 teaches a method and apparatus for measuring strain in structural members utilizing a laser beam to illuminate a surface being analyzed and an optical data digitizer to sense a signal provided by the light beam reflected from the illuminated surface. The optical data digitizer is used to compare the signal received from the surface in a reference condition to subsequent signals received from the surface after surface deformation.
As in known in the art, data from the interference speckle can be used in several ways. While the specimen is stretched, the speckles translate indicating in-plane displacement and also vary in intensity indicating out-of-plane displacement. Due to the nature of materials, it can be assumed that changes from one speckle to an adjacent one are small and therefore linear. Because of this, contour maps of displacements and strains, both in-plane and out-of-plane can be constructed. The mathematical theorems and explanations of the recombination of object and reference beams are known in the art and are further described in a publication of the inventor, Optical Methods of Engineering Analysis, Cambridge University Press 1995, Gary Cloud, which is expressly incorporated herein by reference.
The speckle is itself an interference phenomenon. The formation of speckles in imaging systems can be described at any image region as the superimposition result of the coherent point spread functions for adjacent object points. The speckle created by imaging optics is referred to as a "subjective" speckle. The nature of the illuminated surface gives rise to two different classes of speckle patterns. One class is called the "fully developed" speckle pattern; it develops only from interference of light that is all polarized in the same manner. The speckle field itself will then be similarly polarized. Surfaces at which polarized light is singly scattered, such as matte finished metal, generally give rise to polarized speckle fields as do lightly scattering transmission elements such as ground glass. Matte white paint surfaces or opal glass, into which the light penetrates and is multiply scattered, depolarize the light and thus do not generate a fully developed speckle pattern. The brightness distributions of the two classes of speckle patterns differ substantially, but this difference is not important in the functioning of speckle interferometry systems.
The mixed speckle pattern is recorded by the imaging system. The specimen is then subjected to a load, which causes displacement of the specimen's surface. This displacement causes changes in location and brightness of the various speckles. These newly changed speckles are again recorded by the imaging system. A computer connected to the camera captures the images and calculates displacements and strains on the object's surface based on the changes in the speckle pattern. The actual displacement and strain components that are calculated depend on the configuration of apparatus used, as outlined below.
In one embodiment of the current invention, a uniformly bright field of coherent radiation, which is the so-called reference beam, is added to the speckle field. The addition of the reference field will affect both the size and the brightness distribution of the speckle field. When a reference beam is introduced, the size of a speckle will approximately double. The reason for this involves the interference effect of adding a uniform strong wave to the speckle pattern in the direction of the optical axis.
In this embodiment of the current invention, a beam of coherent light is split. The first portion of the beam is projected onto the surface of the specimen. An imaging lens then collects an image of the speckle pattern formed by the reflection off of the specimen surface. This image is then combined in a beam combiner with the second portion of the beam. The combined image is then captured by a camera, and the computer calculates out-of-plane displacement and strain. This embodiment is used to measure out-of-plane displacement and strain. The data are independent of in-plane displacement as long as the angles of illumination and viewing of the specimen are held in the range 0-10 degrees.
In another embodiment of the invention, a beam of coherent light is split. The first portion of the beam is projected onto the surface of the specimen at a certain angle from the line of viewing. The second portion of the beam is projected onto the surface of the specimen at the opposite angle from the line of viewing. Thus, two beams are used to illuminate the specimen at equal and opposite angles of illumination. The resultant speckle pattern is recorded by the imaging system for before-load and after-load states. The computer calculates in-plane displacement and strain. In this embodiment, the changes in the speckle pattern depend only on in-plane displacement and strain for any angles of incidence, as long as the setup is symmetric with respect to the viewing axis.
The disclosed speckle interferometer system is thus very good at measuring both in-plane and out-of-plane displacements. The setup and function of the two embodiments of the system are different, but both embodiments utilize changes of brightness of individual speckles as the specimen is deformed.
Optionally included is a capability that allows for the regulated change in phase of one of the divided beams. This allows the imaging system to take additional brightness data for a given speckle and calculate precisely the displacement for that speckle without the necessity of creating fringe patterns. As such, the current system provides an efficient non-contacting system that can measure both in-plane and out-of-plane translation of the surface of a specimen.
In view of the above description it is an object of this invention to provide a method and apparatus for measurement of strains in structures of all kinds.
It is further an object of the present invention to measure the relative magnitude of displacements from an original position on different points on a surface of an object under stress.
It is yet another object of the present invention to provide an improved technique and apparatus for performing electronic speckle pattern interferometer in the analysis of motion, strain, and deformation of all kinds of structures, components, bodies and materials.
It is yet another object of the present invention to provide a speckle interferometer which will be useful in engineering, manufacturing, medicine, and natural science to provide precise measurements without the necessity of heavy investment in equipment; and, in its commercial form, can be used by untrained persons in field and industrial environments.
It is yet another object of the present invention to provide an interferometer which is greatly simplified in comparison to traditional setups and is more resistant to vibration and other noises which tend to contaminate the result produced by an electronic speckle pattern interferometer.
It is yet another object of the present invention to provide an interferometer apparatus which affords the capability of placing the illumination sources, (the fiber terminations) on a testing machine or even attaching them to the specimen; while the laser and other components are at a separate locations. In this way, remote measurement of strain in hostile environments can be conducted.
It is yet another object of the present invention to provide an interferometer apparatus which is set up to measure in-plane displacement or strains.
It is yet another object of the present invention to provide an interferometer apparatus that optionally contains collimating lenses for more accurately measuring in-plane or out-of-plane displacement.
It is yet another object of the present invention to provide an interferometer apparatus that contains a phase shifter along one of the fiber optic cables for changing the phase of one of the laser beams to improve strain measurements in the systems.
It is yet another object of the present invention to provide a method and apparatus for the control, analysis and calibration of the phase shifting, the development of displacement maps, determination of specimen displacements, extraction of signal noise from the signal, calculation of strain from displacements maps, and display of displacement and strain maps performed by a computer.
It is yet another object of the present invention to provide an interferometer apparatus utilizing the speckle effect having a phase shifter disposed before the specimen. The speckle interferometer includes a laser, an integrated monobloc adjustable beam splitter and fiber optic coupler, a second adjustable fiber optic splitter, a phase shifter, and a recording medium.
The foregoing as well as other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the appended drawings.